Three-layer mattress spring unit



FIG. 1 is a perspective view of the three-layer mattress spring unit;

FIG. 2 is a right side view thereof, the left side view being a mirrorimage of the right side view;

FIG. 3 is a front view thereof, the rear view being identical to thefront view; and,

FIG. 4 is a top view thereof, the bottom view being identical to the topview.

CLAIM The ornamental design for a three-layer mattress spring unit, asshown and described.